Cr2O3-Al2O3 sintered body and processes for producing same

ABSTRACT

This invention relates to processes for the production of a sintered body consisting of an intimate solid solution of Cr 2 O 3  and Al 2 O 3  by using inexpensive materials, as well as sintered bodies produced thereby. More specifically, it relates to processes for the production of a sintered body which comprises the steps of providing a powder mixture consisting essentially of Cr 2 O 3  powder and Al 2 O 3  powder each having an average particle diameter of not greater than 0.5 μm, or a powder mixture obtained by mixing the aforesaid powder mixture with 1.5 to 15% by weight of TiO 2  powder having an average particle diameter of not greater than 0.5 μm, and sintering the powder mixture at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10 −20  to 10 −5  atm. to yield a sintered body consisting of an intimate solid solution of Cr 2 O 3  and Al 2 O 3  and having a relative density of not less than 95%, as well as sintered bodies produced thereby.

TECHNICAL FIELD

This invention relates to Cr₂O₃—Al₂O₃ ceramics which can be expected to be applicable to high-temperature components subject to severe corrosion by molten salts, such as coatings for the rotor blades of low-quality oil-fired gas turbines, burner nozzles and diffuser cones of crude fuel-fired boilers, and stokers of refuse incinerators, and to processes for producing the same.

BACKGROUND ART

The resistance of Cr₂O₃—Al₂O₃ ceramics to corrosion by molten salts comprising heavy metal oxides and alkali salts depends primarily on the sintering temperature and the content of Cr₂O₃—Al₂O₃ solid solution. In order to achieve high corrosion resistance, it is necessary to form a dense sintered body consisting of a complete solid solution of Cr₂O₃ and Al₂O₃. However, since Cr₂O₃ has poor sinterability, a hot isostatic pressing machine or the like must be used to yield a dense sintered body.

Consequently, conventional processes have the disadvantage that they require a considerable cost and have a limited throughput owing to the use of a hot isostatic pressing machine or the like. Moreover, the conventional processes also have the disadvantage that the formation of a Cr₂O₃—Al₂O₃ solid solution does not proceed to the fullest extent, causing Cr₂O₃ and Al₂O₃ particles to remain in the sintered body.

DISCLOSURE OF THE INVENTION

The present inventors made intensive investigations on processes in which TiO₂ serving as an assistant for accelerating the diffusion of Al₂O₃ and Cr₂O₃ is added in order to improve the sintered density and promote the formation of a Cr₂O₃—Al₂O₃ solid solution, and in which finely powdered raw materials are sintered under conditions including a suitable atmosphere and a suitable temperature. As a result, it has been found that, by adding TiO₂, using finely powdered raw materials, and employing a suitable sintering atmosphere and a suitable sintering temperature, an improvement in sinterability can be achieved to yield a sintered body showing an improvement in compositional homogeneity and resistance to corrosion by molten salts. The constitution of the present invention is as follows.

Embodiment 1

A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr₂O₃ powder having an average particle diameter of not greater than 0.5 μm and Al₂O₃ powder having an average particle diameter of not greater than 0.5 μm, mixing the powder mixture A with TiO₂ powder having an average particle diameter of not greater than 0.5 μm to form a powder mixture B containing the TiO₂ powder in an amount of 1.5 to 15% by weight based on the combined amount of the powder mixture A and the TiO₂ powder, and sintering the powder mixture B at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10⁻²⁰ to 10⁻⁵ atm. to yield a sintered body consisting of an intimate solid solution of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95%.

Embodiment 2

A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr₂O₃ powder having an average particle diameter of not greater than 0.5 μm and Al₂O₃ powder having an average particle diameter of not greater than 0.5 μm, and sintering the powder mixture A at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10⁻²⁰ to 10⁻⁵ atm. to yield a sintered body consisting of an intimate solid solution of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95%.

Embodiment 3

A process for the production of a sintered body as described above in embodiment 2 wherein the sintering temperature is in the range of 1,450 to 1,600° C.

Embodiment 4

A process for the production of a sintered body as described above in any of embodiments 1 to 3 wherein the Cr₂O₃ powder is used in an amount of 40 to 60% by weight based on the powder mixture of Cr₂O₃ powder and Al₂O₃ powder.

Embodiment 5

A sintered body having excellent resistance to corrosion by molten salts which is a sintered body consisting essentially of a solid solution phase composed of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95% and which contains no phase composed of Al₂O₃ alone.

The above-described processes for the production of a sintered body in accordance with the present invention make it possible to produce a dense sintered body consisting of a homogeneous solid solution of Cr₂O₃ and Al₂O₃ and thereby achieve a concomitant improvement in corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the dependence of sintered density on sintering temperature for Cr₂O₃—Al₂O₃ sintered bodies made by adding TiO₂ according to one process of the present invention and a Cr₂O₃—Al₂O₃ sintered body made according to a conventional process;

FIG. 2 is a graph showing the dependence of the half depth of the X-ray diffraction peak recorded with the (1,0,10) plane on sintering temperature for Cr₂O₃—Al₂O₃ sintered bodies made by adding TiO₂ according to one process of the present invention and a Cr₂O₃—Al₂O₃ sintered body made according to a conventional process;

FIG. 3 is a graph showing the dependence of sintered density on the particle diameter of TiO₂ powder for Cr₂O₃—Al₂ O₃ sintered bodies made by adding TiO₂ according to one process of the present invention;

FIG. 4 includes photographs showing the corrosion characteristics of Cr₂O₃—Al₂O₃ sintered bodies made by adding TiO₂ according to one process of the present invention and a Cr₂O₃—Al₂O₃ sintered body made according to a conventional process;

FIG. 5 is a graph showing the dependence of sintered density on sintering temperature for Cr₂O₃—Al₂O₃ sintered bodies made according to another process of the present invention and a conventional process;

FIG. 6 is a graph showing the dependence of the half depth of the X-ray diffraction peak recorded with the (1,0,10) plane on sintering temperature for Cr₂O₃—Al₂O₃ sintered bodies made according to another process of the present invention and a conventional process; and

FIG. 7 includes photographs showing the corrosion characteristics of Cr₂O₃—Al₂O₃ sintered bodies made according to another process of the present invention and a conventional process.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to sintered bodies having excellent resistance to corrosion by molten salts in which its sintered density is increased to 95% or greater and the formation of a Cr₂O₃—Al₂O₃ solid solution is promoted so as to form a single phase. The processes for producing such sintered bodies include a process in which a mixture of Cr₂O₃ powder and Al₂O₃ powder each having a reduced average particle diameter is sintered in a vacuum atmosphere, a process in which a predetermined amount of TiO₂ powder is added to the aforesaid powder mixture and the resulting mixture is sintered in a vacuum atmosphere, and a combination of both processes. These processes are more specifically explained below.

In the process using a powder mixture having a reduced average particle diameter, Cr₂O₃ powder and Al₂O₃ powder each having an average particle diameter of not greater than 0.5 μm are intimately mixed in an organic solvent such as an alcohol (e.g., butanol), acetone, xylene or toluene, and the resulting mixture is sintered in a vacuum atmosphere. The reduced particle diameter of the powder mixture can mitigate the requirements for the diffusion of atoms and thereby promote the formation of a homogeneous solid solution. If the Cr₂O₃ powder has a particle diameter of not greater than 0.2 μm and the Al₂O₃ powder has a particle diameter of not greater than 0.25 μm, a more complete solid solution will be formed easily.

As to the sintering atmosphere, ceramics are generally sintered under atmospheric conditions. However, a vacuum atmosphere must be employed in the present invention.

The reason for this is that, since the oxidation of Cr₂O₃ is promoted under atmospheric conditions and the resulting CrO₃ vaporizes and condenses with other particles in the sintered body, small particles are coarsened to cause a reduction in the density of the sintered body. Accordingly, in order to prevent the oxidation of Cr ion and thereby maintain the density of the sintered body at a high level, a vacuum atmosphere, especially of not greater than 10⁻⁵ atm., is used in the present invention. However, since an unduly high degree of vacuum, especially of less than 10⁻²⁰ atm., may cause Cr ion to be reduced to metallic chromium, it is desirable to use a degree of vacuum in the range of 10⁻⁵ to 10⁻²⁰ atm.

On the other hand, the process involving the addition of a predetermined amount of TiO₂ is based on the principle that the sintered density is raised by utilizing the interaction of TiO₂ with Cr₂O₃ and Al₂O₃. TiO₂ tends to form vacancies for Al and Cr cations under electroneutral conditions. Since the diffusion of atoms proceeds by utilization of these vacancies, Cr₂O₃ and Al₂O₃ become readily soluble in each other and can be easily sintered together. To this end, it is desirable to add TiO₂ powder in an amount of not less than 1.5% by weight. However, since the addition of an unduly large amount of TiO₂ causes the precipitation of a third phase and, on the contrary, provides starting points of corrosion, it is highly desirable to limit the amount of TiO₂ to less than 15% by weight and preferably 4% or less.

In order to increase the diffusion rate of atoms and thereby facilitate sintering, the sintering temperature should be not lower than 1,350° C. However, unduly high temperatures exceeding 1,600° C., on the contrary, tend to cause a reduction in sintered density. Accordingly, it is desirable that the sintering temperature is in the range of 1,350 to 1,600° C. When no TiO₂ powder is added, the number of vacancies for promoting the diffusion of atoms is relatively small. Consequently, it is desirable to employ a somewhat higher sintering temperature in the range of 1,450 to 1,600° C.

The relative density is associated with defects (i.e., cracks) present in the sintered body. At a relative density of less than 95%, through cracks tend to occur frequently, so that corrosive molten salts penetrate deep into these cracks and thereby accelerate corrosion. On the other hand, the occurrence of such cracks is rare at a relative density of 95% or greater, resulting in an improvement in the resistance of the sintered body to corrosion by molten salts. Accordingly, it is desirable that the sintered body has a relative density of not less than 95%.

The contents of Cr₂O₃ powder and Al₂O₃ powder may be in any desired range, so long as they can be homogeneously mixed. However, if there is a large difference in content, one tends to segregate in the other, thus making it difficult to form a solid solution. Accordingly, it is preferable that the content of Cr₂O₃ powder is in the range of 40 to 60% by weight based on the combined weight of Cr₂O₃ powder and Al₂O₃ powder.

EXAMPLES

In order to demonstrate the effects of the present invention more clearly, the following specific examples are given.

Example 1

In this Example 1, there were used Cr₂O₃ powder having an average particle diameter of 0.2 μm, Al₂O₃ powder having an average particle diameter of 0.25 μm, and TiO₂ powder having an average particle diameter of 0.2 or 1.0 μm. Moreover, as a comparative example, a comparison with the present invention was made by carrying out a conventional process using Cr₂O₃ powder having an average particle diameter of 1.0 μm and Al₂O₃ powder having an average particle diameter of 1.0 μm.

Cr₂O₃ powder and Al₂O₃ powder were weighed and mixed so as to give a powder mixture comprising 50 wt. % each of Cr₂O₃ and Al₂O₃. Then, 2.5, 5, or 10 wt. % of TiO₂ powder was added to the above powder mixture, and mixed therewith in an organic solvent (i.e., butyl alcohol) for 120 hours by means of a mixing machine. The resulting mixture was dried at 80° C. with a vacuum dryer. This powder mixture was uniaxially pressed at a pressure of 100 kgf/cm² and then formed into a compact by cold isostatic pressing at a pressure of 1,000 kgf/cm². In an atmosphere having an oxygen partial pressure of 10⁻⁸ atm., this compact was sintered at 1,200-1,600° C. for 3 hours. On the other hand, in the conventional process carried out as a comparative example, Cr₂O₃ powder and Al₂O₃ powder were weighed so as to give a powder mixture comprising 50 wt. % each of Cr₂O₃ and Al₂O₃, and mixed in butyl alcohol for 120 hours by means of a mixing machine. The resulting mixture was dried at 80° C. with a vacuum dryer, pressed in the same manner as described above, and then sintered for 3 hours in a vacuum having an oxygen partial pressure of 10⁻⁸ atm.

With respect to the Cr₂O₃—Al₂O₃ sintered bodies made according to the process of the present invention and the conventional process (comparative example), the dependence of sintered density on sintering temperature is shown in FIG. 1. As shown in FIG. 1, the conventional process failed to give a relative density of 95% or greater at any sintering temperature. In contrast, when the process of the present invention was carried out under such conditions that 2.5 to 10 wt. % of TiO₂ powder having an average particle diameter of 0.2 μm was added, a sintered density of 95% or greater was achieved at a sintering temperature of 1,400° C. However, under such conditions that 20 wt. % of TiO₂ powder having an average particle diameter of 0.2 μm was added, the sintered density did not reach 95% or greater.

Moreover, with respect to 50 wt. % Cr₂O₃—Al₂O₃ sintered bodies made by adding 2.5 wt. % of TiO₂ powder according to the process of the present invention, and a 50 wt. % Cr₂O₃—Al₂O₃ sintered body made according to the conventional process, the dependence of the half width of the X-ray diffraction peak recorded with the (1,0,10) plane of the Cr₂O₃—Al₂O₃ solid solution on sintering temperature is shown in FIG. 2. As shown in FIG. 2, when the process of the present invention was carried out under such conditions that 2.5 wt. % of TiO₂ powder was added, the peak of the resulting Cr₂O₃—Al₂O₃ solid solution was sharpened as compared with the sintered body made according to the conventional process, indicating that a more homogeneous composition was obtained.

With respect to Cr₂O₃—Al₂O₃ sintered bodies having TiO₂ powder added thereto, the dependence of sintered density on the particle diameter of TiO₂ powder is shown in FIG. 3. As shown in FIG. 3, when the sintering temperature was 1,450° C., TiO₂ powder having a particle diameter of 1 μm gave a sintered density of 94%, but TiO₂ powder having a particle diameter of 0.2 μm gave a sintered density of 98%. Thus, a higher sintered density is obtained by using TiO₂ powder having a particle diameter of 0.2 μm. It is believed that, since the sintered density depends on the dispersed state of TiO₂, finer TiO₂ powder is more uniformly dispersed and hence causes an enhancement in sintered density. Accordingly, the average particle diameter of TiO₂ powder has been determined to be not greater than 0.5 μm as described above.

Furthermore, with respect to Cr₂O₃—Al₂O₃ sintered bodies made by adding TiO₂ powder having an average particle diameter of 0.2 μm according to the process of the present invention, the depth of erosion caused by soaking them in a molten 60 wt. % V₂O₅−40 wt. % Na₂SO₄ salt at 900° C. for 100 hours is shown in FIG. 4. At the same time, with respect to a 50 wt. % Cr₂O₃—Al₂O₃ sintered body made according to the conventional process, the depth of erosion caused by soaking it in a molten 60 wt. % V₂O₅−40 wt. % Na₂SO₄ salt at 900° C. for 100 hours is also shown in FIG. 4 as a comparative example. For the sintered bodies made according to the process of the present invention, those having a Cr₂O₃ content of 40-60 wt. % showed a depth of erosion of not greater than 100 μm and hence had higher corrosion resistance as compared with the sintered body made according to the conventional process and showing a depth of erosion of 400 μm. Thus, when sintered bodies are made according to the process of the present invention, they have a higher sintered density and a more homogeneous composition, and hence show an improvement in resistance to corrosion by molten salts.

Example 2

In this Example 2, there were used Cr₂O₃ powder having an average particle diameter of 0.2 μm and Al₂O₃ powder having an average particle diameter of 0.25 μm. Moreover, a comparison with the present invention was made by carrying out a conventional process using Cr₂O₃ powder having an average particle diameter of 1 μm and Al₂O₃ powder having an average particle diameter of 1 μm.

Cr₂O₃ powder and Al₂O₃ powder were weighed so as to give a powder mixture comprising 50 wt. % each of Cr₂O₃ and Al₂O₃, and mixed in butyl alcohol for 120 hours by means of a mixing machine. The resulting mixture was dried at 80° C. with a vacuum dryer. This powder mixture was uniaxially pressed at a pressure of 100 kgf/cm² and then form into a compact by cold isostatic pressing at a pressure of 1,000 kgf/cm². In an atmosphere having an oxygen partial pressure of 10⁻⁵ atm., this compact was sintered at 1,300-1,600° C. for 3 hours. On the other hand, in the conventional process carried out as a comparative example, Cr₂O₃ powder and Al₂O₃ powder were weighed so as to give a powder mixture comprising 50 wt. % each of Cr₂O₃ and Al₂O₃, and mixed in an organic solvent for 120 hours by means of a mixing machine. The resulting mixture was dried at 80° C. with a vacuum dryer, pressed in the same manner as described above, and then sintered for 3 hours in a vacuum having an oxygen partial pressure of 10⁻¹⁵ atm.

With respect to the Cr₂O₃—Al₂O₃ sintered bodies made according to the process of the present invention and the conventional process, the dependence of sintered density on sintering temperature is shown in FIG. 5. As shown in FIG. 5, the conventional process failed to give a relative density of 95% or greater at any sintering temperature. In contrast, the process of the present invention yielded a sintered body having a sintered density of 98% or greater at a sintering temperature of 1,500° C. Moreover, with respect to the Cr₂O₃—Al₂O₃ sintered bodies made according to the process of the present invention and the conventional process, the dependence of the results of X-ray diffraction analysis on sintering temperature is shown in FIG. 6. As can be seen from this figure, when the process of the present invention was employed, the peak of the resulting Cr₂O₃—Al₂O₃ solid solution was sharpened as compared with the sintered body made according to the conventional process, indicating that a more homogeneous composition was obtained.

Moreover, with respect to sintered bodies made according to the process of the present invention and hence having a sintered density of 95% or greater and a homogeneous composition, and a sintered body made according to the conventional process, the depth of erosion caused by soaking them in a molten 60 wt. % V₂O₅−40 wt. % Na₂SO₄ salt at 900° C. for 100 hours is shown in FIG. 7. For the sintered bodies made according to the process of the present invention, those having a Cr₂O₃ content of 40-60 wt. % showed a depth of erosion of 150 μm as contrasted with the sintered body made according to the conventional process and showing a depth of erosion of 400 μm. Thus, when sintered bodies are made according to the process of the present invention, they have a higher sintered density and a more homogeneous composition, and hence show an improvement in resistance to corrosion by molten salts.

Exploitability in Industry

As described above, the present invention may be applied to high-temperature components subject to severe corrosion by molten salts, such as coatings for the rotor blades of low-quality oil-fired gas turbines, burner nozzles and diffuser cones of crude fuel-fired boilers, and stokers of refuse incinerators. 

What is claimed is:
 1. A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr₂O₃ powder having an average particle diameter of not greater than 0.5 μm and Al₂O₃ powder having an average particle diameter of not greater than 0.5 μm, mixing said powder mixture A with TiO₂ powder having an average particle diameter of not greater than 0.5 μm to form a powder mixture B containing the TiO₂ powder in an amount of 1.5 to 15% by weight based on the combined amount of said powder mixture A and the TiO₂ powder, and sintering said powder mixture B at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10⁻²⁰ to 10⁻⁵ atm. to yield a sintered body consisting of an intimate solid solution of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95%.
 2. A process for the production of a sintered body having excellent resistance to corrosion by molten salts which comprises the steps of providing a powder mixture A consisting essentially of Cr₂O₃ powder having an average particle diameter of not greater than 0.5 μm and Al₂O₃ powder having an average particle diameter of not greater than 0.5 μm, and sintering said powder mixture A at a temperature of 1,350 to 1,600° C. in a vacuum atmosphere of 10⁻²⁰ to 10⁻⁵ atm. to yield a sintered body consisting of an intimate solid solution of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95%.
 3. A process for the production of a sintered body as claimed in claim 2 wherein the sintering temperature is in the range of 1,450 to 1,600° C.
 4. A process for the production of a sintered body as claimed in any of claims 1 to 3 wherein the Cr₂O₃ powder is used in an amount of 40 to 60% by weight based on the powder mixture of Cr₂O₃ powder and Al₂O₃ powder.
 5. A sintered body having excellent resistance to corrosion by molten salts which is a sintered body consisting essentially of a solid solution composed of Cr₂O₃ and Al₂O₃ and having a relative density of not less than 95% and a Cr₂O₃ and Al₂O₃ solid solution present as a single phase. 